The present invention relates to a seismic prospection method in which the converted waves are processed.
The general principle of seismic prospection consists in using a seismic source to create a disturbance in the subsoil and in using sensors to record seismic data generated by the disturbance so as to extract information therefrom about the geology of the subsoil, and in particular to detect the presence of hydrocarbons.
FIG. 1 shows a sound wave propagating in the subsoil from a source 1. The sound wave in the example shown is a compression wave which is reflected in the subsoil to give rise to components comprising a compression reflected wave and a shear reflected wave.
Compression waves (so-called P-waves) vibrate in their direction of propagation, while shear waves (so-called S-waves) vibrate perpendicularly to their direction of propagation. The propagation speed of shear waves is slower than the propagation speed of compression waves, and knowledge of the speed fields of compression waves and of shear waves can be used to determine information about the subsoil. For example, the ratio between the speeds of the compression waves and of the shear waves can be used to determine the pressure coefficient of the rocks they have traveled through and can also be used as an indicator of hydrocarbon presence.
Conventionally, in order to invert seismic data, speed field models are used which depend on various parameters that are assumed to be invariant over a given range of source-receiver offsets and in a given three-dimensional acquisition zone, however these parameters can vary xe2x80x9cslowlyxe2x80x9d in three dimensions, i.e. they can differ from one zone in three dimensions to another.
To invert seismic data corresponding to SS or PP reflections in the subsoil, parameters Vp and Vs are used which represent the apparent speeds of the compression waves and of the shear waves after dynamic correction (xe2x80x9cnormal move outxe2x80x9d or xe2x80x9cNMOxe2x80x9d), and also parameters Tp and Ts which represent respectively the vertical travel times of the P-waves and of the S-waves. The parameters Tp and Vp suffice for PP speed analysis, while the parameters Ts and Vs suffice for SS speed analysis.
Converted speeds (PS reflections) are generally analyzed by using models in the time domain which make use of the parameters Vp and Vs, and also of a parameter Vc where Vc is such that:
Tcxc2x7Vc2=Tpxc2x7Vp2+Tsxc2x7Vs2 where Tc=Ts+Tp
The models making use of those three parameters are effective with materials that are homogeneous and isotropic for S-waves and P-waves. However, in media that are vertically inhomogeneous or that are highly anisotropic, it has been shown that account needs to be taken of two other parameters, referred to in the literature as xcex3eff and xcex30, where xcex3eff=xcex3n2/xcex30 with xcex3n=Vp/Vs and xcex30=Ts/Tp.
The offset X between the reflection point and a source depends to the first order on the parameter xcex3eff and to the second order on the parameter xcex30, and also on the quantity Tcxc2x7Vc2.
In this respect, reference can advantageously be made to the following publication:
[1] L. Thomsen, 1998, xe2x80x9cConverted-wave reflection seismology over anisotropic, inhomogeneous mediaxe2x80x9d, 68th Annual Meeting, SEG Expanded Abstracts, pp. 2048-2051.
Nevertheless, in that publication, the parameter xcex3eff is assumed to be known. Unfortunately, in practice and as a general rule, none of the above-mentioned parameters is known immediately.
An object of the invention is to provide a seismic processing method applicable to converted waves which is particularly reliable and independent of any prior knowledge of the parameters xcex3eff and xcex30.
Proposals have recently been made to determine the lateral offset of the conversation point by using the lateral correlation between forward source-receiver offset images and backward source-receiver offset images, i.e. images obtained by inverting the positions of the sources and of the receivers.
In this respect, reference can be made to:
[2] P. Hermann, G. Michaud, P. Y. Granger, 1999, xe2x80x9cStacking mode-converted wavesxe2x80x9d, presented at the CSEG Conference, Calgary, May 1999.
The invention provides a seismic prospection method in which a compression seismic wave is emitted into the subsoil and sensors are used to collect seismic data having at least a shear component, and in which the data corresponding to said shear component is processed to deduce information about the geology of the subsoil, the method being characterized in that an estimate of the ratio:       ∫          Z      0        Z    ⁢            v      p        ·          xe2x80x83        ⁢                  ⅆ        l            /                        ∫                      Z            0                    Z                ⁢                              v            s                    ·                      xe2x80x83                    ⁢                      ⅆ            l                              
is determined where vp and vs are values for real local compression and shear speeds, where l is the depth coordinate in the subsoil, where Z is the value of this depth coordinate at the bottom surface of the last layer to be analyzed and where Z0 is the value of this depth coordinate at the top surface of said layer or of a layer above it, and the seismic data is inverted in order to deduce the local values of compression and shear speed for said layer to be analyzed, by using a model in which this estimate is used for the invariant parameter xcex3eff.
The invention advantageously further includes the following characteristics taken singly or in any technically feasible combination:
the parameter xcex3eff is determined for various different possible values thereof by applying migration processing to the seismic data that corresponds to the shear component, and by determining the value for the parameter xcex3eff at which the forward and backward seismic images are best correlated;
to vary the parameter xcex3eff, the following notation is used:
vpxcex1=xcex1vp0 and vsxcex2=xcex2vs0
where vp0 and vs0 are previously determined approximate values for vp and vs, and
both of the variables xcex1 and xcex2 are varied;
the model uses as invariant parameters at least four of the following parameters: xcex30, xcex3eff, Tp, Fp, Tc, and Fc where xcex30=Ts/Tp, xcex3eff=Fp/Fs, Tc=Tp+Ts, and where Tp and Ts represent the vertical travel times for the compression and shear waves respectively, where Fp is such that (Fp/Tp)1/2 represents a compression speed, and where Fc is such that ((Fcxe2x88x92Fp)/Ts)1/2 represents a shear speed;
when the variables xcex1 and xcex2 are varied, the parameters xcex30, xcex3eff, Tp, Fp, and Fc are replaced as follows:
xcex30xe2x80x2=xcex1/xcex2*xcex30
xcex3effxe2x80x2=xcex1/xcex2*xcex3eff
Tpxe2x80x2=Tp*(1+xcex30)/(1+xcex30xe2x80x2)
Fpxe2x80x2=Fp*xcex12*(1+xcex30)/(1+xcex30xe2x80x2)
Fcxe2x80x2=Fc*xcex1xcex2*(1+xcex30)/(1+xcex30xe2x80x2)*(1+xcex3effxe2x80x2)/(1+xcex3eff)
and migration is applied to the seismic data corresponding to these new parameters;
to vary the parameter xcex3eff, xcex2 is set equal to 1/xcex1, and xcex1 is varied;
after the parameter xcex3eff has been determined, vp and vs are varied while keeping xcex3eff constant, and the parameter Fc is determined for which the alignment in the offset axis is at a maximum;
to vary vp and vs, the following notation is used:
vpxcex1=xcex1vp0 and vsxcex1=xcex1vs1
where vp1 and vs1 are values determined for vp and vs in step 2, and the variable xcex1 is varied;
after determining the parameter Fc, the parameter Tp and/or the parameter xcex30 =Ts/Tp is/are determined;
the parameter Tp is advantageously determined from the vp speed field determined by analyzing the compression component of the seismic data;
processing is subsequently performed to bring the S-speed and the P-speed models to a common depth; and
after processing to achieve a common depth, large-offset curvature processing is implemented by varying the anisotropy parameters xcex4 and "sgr" while keeping the following ratio constant:
(1+2xcex4)/(1+2"sgr")